Inhibited nitrogen tetroxide with fluorine-containing oxidizer



United States Patent Ofi 3,507,717 Patented Apr. 21, 1970 ice 3,507,717 INHIBITED NITROGEN TETROXIDE WITH FLUORINE-CONTAINING OXIDIZER Hubert E. Dubb, Sepulveda, Ralph C. Greenough,

Los Angeles, and Bartholomew L. Tuflly, Woodland Hills, Califi, assignors to North American Rockwell Corporation No Drawing. Filed Apr. 2, 1965, Ser. No. 445,871 Int. Cl. C01b 21/36; C06d /00 US. Cl. 149-1 8 Claims This invention relates to a method and additives used to enhance the storage capability of nitrogen tetroxide. More specifically, the invention relates to a method and additives utilized to effectively remove water or its equivalency from nitrogen tetroxide to reduce its corrosive tendencies.

Nitrogen tetroxide, NTO, is one of the most predominantly utilized rocket fuel oxidizers. Successful utilization of NTO has not been entirely realized due to the problem of corrosivity. When NTO is dry, that is essentially free of moisture content, the problem does not result. However, when moisture is present, NTO then becomes an extremely corrosive compound. The corrosivity results from the formation of nitric and nitrous acids through the reaction of the NTO with water present. Because of this problem, military specifications have set the maximum water equivalency to be .14 weight percent of the NTO composition.

It had been previously assumed that NTO would present no serious storage or handling problem if it was stored and transferred in clean, air-tight, moisture-free systems handled by trained personnel. However, corrosion effects brought to light the problem of moisture content in NTO. As a result of the moisture problem there have been material failures as well as erosion phenomena due to the reaction of the moist oxidizer with the environment. Due to corrosion of container materials and thus the resultant erosion, a solid phase is introduced into the system. This is in the form of a sludge which causes clogged filters, flow decay rates in metering equipment, intermittent irregularities, and unexpected residue appearances. Additionally, due to the corrosion there occurs premature hardware failures, leaks, and sporadic Weakening of auxiliary equipment because of rapid localized attack such as pitting and intergranular invasion.

The main problem with water content is that the NTO reacts with the moisture to form HNO and HNO The HNO then further reacts to form water and N 0 The HNO reacts with valve and container materials to form wall nitrate, water and NO or N 0 The H O then reacts with more NTO to reform HNO The net result is a catalytic nitration of the materials of construction with an increase in N 0 but no decrease in the HNO content of the NTO. The NTO solution when nitrating with the wall thus loses none of its corrosivity; due to the reintroduction of water into the system. Following is a sequence of the above reactions:

In overall form the above reactions can be summarized as follows:

H20 2N204 "Wall Wa11++ ZNOf-i- 2N0 Thus it is an object of this invention to provide a method for inhibiting the corrosivity of moisture laden NTO. Another object of this invention is to describe a method and additive for effectively removing water present in NTO.

The above and other objects of the invention are accomplished by adding to the moisture laden NTO in an amount sufiicient to convert all the water or its equivalent present in the NTO, a fluorine-containing oxidizer compound which forms at least HF as a reaction product with said water or its equivalent compound and which oxidizer compound is further non-destructive with regard to the NTO to which it is added. The formation of the HF is desired since it is believed that the compound forms a protective coating on the Walls of the container in which the NTO was stored which further inhibits possible corrosion. It is pointed out that the formation of HF, however, will cause the solution to corrode certain materials which are not commonly used for storage of NTO. The protective coating and lack of corrosion are present when NTO with the additives of this invention are utilized in aluminum, for example, which is a most preferable storage material because of its low cost, easy machinability and light weight. Some costly materials that are more difl'icult to machine and are of greater density than aluminum, such as Inconel, are not affected by moisture laden NTO. However, military specifications, as pointed out, require a minimum water equivalency because of the desirability of using aluminum for storage.

A particular example of an oxidizer compound that has been found to successfully remove moisture content in NT 0 is F NO. Though theoretically water could be removed from NTO by distillation processes, the presence of an additional compound as contemplated by this invention acts as an inhibitor that is continually present to react with any water that is later introduced into the NTC through handling, storage and the like. Thus, the addition of the compound provides a continuous protection against the corrosive effects of moisture. One of the particular advantages of F NO is that it reacts with nitric acid to form both HF and regenerate N 0 as indicated by the following reactions:

The overall reaction with H2O is as follows:

The N 0 (NO+NQz) in the above reaction is normally present in any NTO shipped from a manufacturer or is present due to reaction (2); as can be seen in all the above reactions, N 0 is'always regenerated. Of course the regeneration of the NTO is peculiar to the utilization of the F NO used as the additive material. When other fluorine-containing oxidizers are utilized the regeneration of NTO might not occur. However, as long as the reaction products do not deleteriously affect the NTO as far as performance is concerned nor in any other Way act deleteriously, then they can be also successfully utilized.

Examples of other fluorine-containing oxidizers which may be utilizable in removing the moisture or water or its equivalent from NTO are ClF C11 P N E; and the like. Through experimentation by the addition of fluorine oxidizers to moisture laden NTO one can determine if the moisture present has been converted to HF which forms the desired wall coating and other nonreactant products which do not disturb the oxidizing capability of the NTO. The amount of additive to be utilized is determined by the stoichiometric quantity required to react with the water equivalency present in the NTO in accord with reaction (6), for example. It is preferable to generally double the stoichiometric quantity in order to have an excess of additive present to further react with any moiture that should later get into the system. The following are a series of specific examples which indicate the effectiveness of the method of the invention in removing moisture from NTO.

EXAMPLE I Two hundred milliliters of a sample of NTO obtained from a supplier were placed in an enclosed steel container. A second 200 milliliters of the same NTO was placed in a second identical container. The water equivalency in the NTO was determined by dividing the 200 milliliter sample in half and running standard Turner evaporative tests in Pyrex apparatus. The water equivalency was found to be .15 weight percent. In order to meet US. Government specifications the water equivalency must be less than 0.14 weight percent. To the second 200 milliliter sample of NTO was added F NO in a sufiicient amount to equal 2.3 weight percent of the solution. This amount of F NO is equivalent to 4.8 times the stoichiometric amount required to react with the water equivalency present in the NTO. This sample was divided into two equal 100 milliliter portions and standard military specification evaporative tests were run on each portion. The tests were run in a standard Pyrex equipment. The military test designation for determining water equivalency is P-26539A; this test has been shown to be reproducible to :0.01 percent. The sample containing no additive, of course, maintained the same water equivalency in both measurements. However, the sample to which the F NO was added displayed in both portions a water equivalency of .06 weight percent less than half that originally present. Thus the NTO went from a state where it was not within military specifications to one where it was easily within such specifications. The small water equivalency remaining is believed to be due solely to the reaction of the HF generated with the glass test apparatus. The glass represented by silica, SiO reacts with HP to form insoluble fiuorosilicates, SiF and H 0. Visual observation of the glass apparatus indicated a slight etching, thus is was reasonable to assume that the water equivalency present was due solely to these reactions. Thus when the material is to be stored in a metal container there should be absolutely no water equivalency present in NTO solutions.

EXAMPLE II The procedure of Example I was repeated with NTO originally having a water equivalency of .09. Upon the addition of F NO in an amount equal to 1.5 weight percent of the solution, the water equivalency then dropped to .03. A third sample having .09 water equivalency was utilized. To the sample was added 1.28 percent F NO with a resultant water equivalency of .02. Thus, though a lesser amount of F NO was added, the water equivalency was lower, indicating that the etching reaction may have been not as severe as with the previous sample. This slight difference (0.03 v. 0.02) is of the same order of magnitude as the reproducibility of the tests and can thus not be considered significant.

EXAMPLE III The procedure of Example I was repeated with 311.9 grams of NTO solution having a water equivalency initially of .08 percent. Upon the addition of 3.12 grams of 0P the water equivalency was found to be reduced to .04 percent.

EXAMPLE IV The procedure of Example 1 was repeated with 443.2 grams of NTO solution having a water equivalency initially of .08 percent. Upon the addition of 5 grams of N 11 the water equivalency was found to be reduced to .03 percent.

EXAMPLE V In order to test metal samples in the NTO solutions, a Teflon bomb was utilized. The NTO is inert with regard to Teflon. The small angular flats of metal used as the test samples were suspended into the NTO liquid by Teflon wire, thus the inert Teflon was the only extraneous material with which the metal sample and the liquid NTO were in contact.

The metal flats were cleaned carefully with suitable solvents and (or) abrasives, rinsed with distilled water and acetone, and carefully dried. Weights of the metal specimens were obtained to the nearest 0.1 milligram. Transference of the NTO solutions was done as rapidly as possible by pouring in a dry box where the tops of the Teflon bombs containing the metal specimens were screwed on. An aluminum sample initially weighing 2.5016 grams was suspended in an NTO solution having a water equivalency of 0.09 weight percent. The temperature of the solutions was maintained at 25 C. for 18 days. Upon removal at the end of the time period, the weight was determined to be 2.4986 grams. Thus, the sample in the uninhibited NTO had a loss of weight of .0030 gram. The same procedure was repeated with NTO also originally having 0.09 weight percent H O to which 1.28 weight percent of F NO had been added. The aluminum sample originally weighed 2.4988 grams. The solution was maintained at 25 C. for 43 days. On removal of the aluminum sample at the end of the time period, the weight was found to be 2.4967 grams. Thus, the aluminum sample in the inhibited NTO lost only .0021 gram in a time period more than twice as long as the uninhibited one. It can thus be seen that the presence of the additives of this invention, specifically F NO in this instance, significantly decreases the corrosive effect of moisture laden NTO.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

We claim: 1. A method of decreasing the water equivalency content of nitrogen tetroxide composition having water or its equivalent compounds as an impurity mixed therewith comprising:

adding to said nitrogen tetroxide composition a fluorine-containing oxidier selected from the group consisting of F NO, 0E and N 1 in at least an amount sufiicient to convert said water impurity to HF and other compounds compatible with said nitrogen tetroxide.

2. The method of claim 1 wherein said fluorinecontaining oxidier is BN0.

3. The method of claim 1 wherein said fluorine-containing oxidizer is 0P 6 4. The method of claim 1 wherein said fiuorine-con- 7. The composition of claim 5 wherein the oxidizer is taining oxidizer is N h. 0H,.

5. A composition of matter comprising: 8. The composition of claim 5 wherein the oxidizer is nitrogen tetroxide having water equivalent impurities z 4- therein, and a fluorine oxidizer selected from the 5 References Cited groun consisting of F NO,, OF: and N 1 which forms at least HF as one product of its reaction UNITED STATES PATENTS with said impurities in at least an amount sufiicient 3,053,880 11/ 1962 Degel' et X to react with substantially all of said Water equivalent impurities, and wherein any other products of 10 OTHER REFERENCES said reaction are non-destructive toward nitrogen ROSS: Journal Of the Amfil'ican Rocket Society, tetroxide. 80, March 1950, pp. 2431.

Fagio'lhe composltlon of claim 5 wherein the oxidizer IS BENJAMIN R- PADGETI, Primary Examiner 

1. A METHOD OF DECREASING THE WATER EQUIVALENCY CONTENT OF NITROGEN TETROXIDE COMPOSITION HAVING WATER OR ITS EQUIVALENT COMPOUNDS AS AN IMPURITY MIXED THEREWITH COMPRISING: ADDING TO SAID NITROGEN TETROXIDE COMPOSITION A FLUORINE-CONTAINING OXIDIER SELECTED FROM THE GROUP CONSISTING OF F3NO, OF2 AND N2F4 IN AT LEAST AN AMOUNT SUFFICIENT TO CONVERT SAID WATER IMPURITY TO HF AND OTHER COMPOUNDS COMPATIBLE WITH SAID NITROGEN TETROXIDE. 